1. Field of the Invention
The present invention relates to a liquid crystal display panel composed of picture elements arranged in matrix form having a liquid crystal element and a ferroelectric element and to a method for driving it.
2. Description of the Prior Art
Several liquid crystal display panels are known, in which only liquid crystal picture elements are arranged in matrix form. However, by using such liquid crystal display panels, it is difficult to display moving pictures for the reason (1) that the response speed is low, (2) that no response is obtained by a single pulse and (3) that no satisfactory response is obtained by using a driving equipment having a small duty ratio. In order to overcome these drawbacks, there has been proposed a variety of display devices comprising liquid crystal and non-linear elements in combination.
For example, in U.S. Pat. No. 3,725,899, a liquid crystal display panel is disclosed, in which picture elements having two or three terminals using a ferroelectric material such as a non-linear element are arranged in matrix form. With a liquid crystal display penel composed of two terminal picture elements it is not possible to improve the response speed considerably, while a liquid crystal display panel composed of three terminal picture elements has the drawback in that its structure is so complicated that, in a large scale device, it is not possible to completely prevent cross talk.
In order to overcome the drawbacks mentioned above, two of the inventors of the present invention have proposed, in Japanese patent application No. Sho 47-36914 (Publication No. Sho 49-3649), a liquid crystal display panel, in which two terminal picture elements, arranged in a matrix form of large scale without crosstalk and at the same time having a high response speed, comprising a liquid crystal element and a ferroelectric element connected in series and a capacitor of large capacitance connected in parallel with the liquid crystal element (hereafter this method for improving the characteristics of liquid crystal picture elements is called the "capacitor addition method") are arranged in a matrix form. A liquid crystal picture element according to the capacitor addition method functions in four steps: writing, first discharge, erasing, and second discharge. For writing a potential of positive polarity, which is greater than the coersive force of the ferroelectric element, is applied to the two terminals of the picture element. Since the capacitance of the ferroelectric element is negligibly small with respect to that of the added capacitor, the potential is applied amost entirely to the ferroelectric element and switches its polarization. The added capacitor is charged due to the polarization. During the step of the first discharge, the capacitor is discharged through the liquid crystal element and the latter scatters light. For the erasing step, a potential of negative polarity is applied to the two terminals, in order to return the polarization to its initial state and the capacitor is charged again. After the erasing step, the capacitor is discharged again and the liquid crystal element scatters light. The above mentioned patent application has proposed only a high response speed picture element having two terminals and has not resolved how the number of such picture elements comprising capacitors of large capacitance can be arranged in a matrix form of large scale.